The goal of these experiments is to identify how cortical sheet size contributes to cortical field formation during early development. Previous work reducing the size of the cortical sheet has indicated that the cortical sheet is not field specific early in development (Huffman et al., 1999a). This study will examine what happens to cortical fields when the cortical sheet is enlarged, a phenomena that has occurred throughout evolution in a number of lineages, including primates. Specifically, we hypothesize that disproportionately increasing the neocortex will increase the size of existing cortical fields and/or create a new cortical field. The first phase of this study will be to enlarge the cortex in developing short-tailed opossums and to quantify the changes that occur in cortical sheet size. The second phase will be to examine cortical organization using electrophysiological recording techniques and myeloarchitecture. The final phase will be to examine thalamocortical and corticocortical connections. Knowing whether the correlation between cortical sheet size and number of cortical fields can be reproduced solely by altering cortical sheet size will help us understand the mechanisms underling the evolution of complex brains. Further, these experiments will provide general insight into human disorders that affect cortical size, connectivity, and functional organization during development. [unreadable] [unreadable] [unreadable]